CN111919296A - Power module and method of manufacturing power module - Google Patents

Abstract

Disclosed is a power module (1), the power module (1) comprising: -a first and a second substrate (10), each substrate comprising a patterned layer (12) of electrically conductive material; -a plurality of prepackaged power cells (20) positioned between the substrates, each cell comprising: -an electrically insulating core (21) in which at least one power die (21) is embedded, and-two outer layers (23) of electrically conductive material on opposite sides of the electrically insulating core (21), said outer layers being connected to respective patterned layers of the substrate, respectively, wherein each outer layer of the pre-packaged power unit comprises a contact pad (230) connected to a respective contact (220) of the power die by a connection arranged in the electrically insulating core (21), said contact pad having a surface area larger than the surface area of the power die electrical contact connected thereto.

Description

Power module and method of manufacturing power module

technical field

The present invention relates to a power module comprising two substrates and a plurality of prepackaged power cells positioned between the substrates, each prepackaged power cell comprising at least one power die. The invention also relates to a method of manufacturing such a power module.

Background technique

For example in many different fields (eg in the automotive, aviation, railway industry) power dies such as diodes or various types of power transistors (MOSFET, JFET, IGBT, HEMT) are used for the control and conversion of electrical power Basic components of a power module. The power die is constructed vertically as opposed to the signal conditioning die, which is typically constructed in a lateral manner. As a result, the die has electrical connections on its sides.

The most common way to connect power dies to other components (eg, in power modules) today is through the use of a substrate such as a direct bond copper (DBC) substrate, which includes a ceramic board covered with a copper layer on at least one side. One side of the power die is soldered or sintered to the substrate, and the other side of the die is connected by wire bonds or ribbons ultrasonically soldered onto the die metallization.

The ever-increasing switching frequency allows the overall converter to be reduced in size. This miniaturization results in the concentration of power in a reduced volume. Therefore, the heat density is further increased. All of these evolutions converge to require increased reliability and thermal enhancement of power modules.

To enhance heat dissipation in power modules including multiple power dies, power modules have been proposed that include two cooling substrates (eg, DBCs) between which the power dies are sandwiched. This is the case, for example, in document US 6,072,240, which discloses a plurality of IGBT dies positioned between two substrates.

The main advantage of these power modules is their good ability to dissipate heat through double-sided cooling.

However, existing double-sided cooling solutions are not without drawbacks. First, using sintering techniques, bonding the die between two substrates requires applying pressure on the substrate and on the die. Since the dies are very fragile, care must be taken to properly distribute the pressure over the surface of the substrate so as not to damage any dies positioned in between.

Second, die thickness typically varies between various types of power dies (eg, between diodes and power switches). Due to this thickness difference, the second substrate cannot be directly bonded to the upper side of the die. To accommodate these thickness variations, solutions have been proposed in which the substrate is non-planar and exhibits raised areas or pillars (eg in document US2011/0254177). However, the design and manufacture of such modules is complicated due to the dimensional tolerances of the individual parts.

Power modules are also proposed in which the interface between the die and the second substrate is formed by sintering bumps on the die and the substrate, the size of the bumps adapting to thickness variations between the dies. In this solution, the bumps also act as spacers in order to ensure functional electrical isolation. If the substrate is separated only by the thickness of the die (about 0.5mm on average), functional insulation cannot be ensured. Additionally, the added pads need to be soldered on the die and on the substrate, thereby increasing the solder layer, which increases thermal stress and degrades thermal conductivity.

However, these solutions do not address another problem, which is the very limited size of the electrical contacts of the power die. For example, transistor control pads (gate or base) are much smaller than power pads. Therefore, connecting it to the substrate is particularly difficult. Especially for wide-bandgap devices, gate connections require high-precision parts and accurate alignment, which in turn involves expensive equipment and risks high yields.

Given this problem, the above solutions are not currently commercially available, and the double-sided modules seen in the literature are limited to large feature sizes like IGBT chips or diodes, and are not suitable for any type of power die.

In the publication "Design of a novel, high-density, high-speed 10kV SiC MOSFET module" by C. DiMarino et al. (2017 IEEE Conference and Exhibition on Energy Conversion (ECCE), 2017, pp. 4003-4010), by expanding The gate pad to allow connection with the molybdenum pad solves the problem of the size of the gate pad of the SiC MOSFET. However, this solution requires extensive modifications to the die metallization, thereby compromising efficient heat and current transfer.

SUMMARY OF THE INVENTION

In view of the above, the present invention aims to provide a power module with efficient cooling and high reliability.

Another object of the present invention is to provide a power module with double-sided cooling whose die attach is easier while ensuring the required electrical isolation between opposing substrates.

Another object of the present invention is to provide a power module that is easier to manufacture than the prior art.

Therefore, a power module is disclosed that includes:

- a first planar substrate and a second planar substrate, each substrate comprising a layer of thermally conductive material and a patterned layer of electrically conductive material,

- a plurality of prepackaged power cells positioned between the first planar substrate and the second planar substrate, each prepackaged power cell comprising:

ο Electrically insulating cores,

o at least one power die embedded in an electrically insulating core, each power die having opposing electrical contacts, and

o two outer layers of conductive material on opposite sides of the electrically insulating core, said outer layers being respectively connected to the respective patterned layers of conductive material of the planar substrate,

wherein each outer layer of conductive material of the prepackaged power cell includes contact pads connected to corresponding electrical contacts of the power die by connections arranged in the electrically insulating core of the prepackaged power cell, the surface area of the contact pads being greater than the surface area of the electrical contacts of the power die to which it is connected.

In an embodiment, each prepackaged power cell further includes two inner layers of conductive material embedded in the electrically insulating core, each inner layer being positioned between the power die and a corresponding outer layer, the outer layers having a thickness greater than that of the inner layer thickness,

And the connection between the contact pads of the outer layer and the corresponding electrical contacts of the power die includes a first connection between the contact pads of the outer layer and the contact pads of the corresponding inner layer and the connection of the corresponding inner layer. A second connection between the contact pads and corresponding electrical contacts of the power die.

The surface area of the contact pads of the inner layer of conductive material may then be greater than the surface area of the electrical contacts of the power die to which they are connected.

The connections between the outer and inner layers of the prepackaged power cells and the connections between the inner layers of the power die and the electrical contacts may be through holes arranged in the electrically insulating core.

In an embodiment, the power module further includes a layer of electrically and thermally conductive bonding material selected from the group consisting of solder paste, sinter paste, or conductive paste between each outer layer of the prepackaged power cell and the patterned layer of the substrate.

According to an embodiment, the power module may further include a dielectric material filling the spaces between the substrates and the spaces between the prepackaged power cells.

Preferably, the at least two power dies contained in the different prepackaged power cells have different thicknesses measured as the maximum distance between electrical contacts on opposite sides of the power die, and the corresponding prepackaged power cells have in their Equal thickness measured between respective two outer layers of conductive material. The planar substrate may be a direct bonded copper substrate, an insulated metal substrate, or a reactive metal brazed substrate.

In an embodiment, the power module further includes at least one passive component mounted on the patterned layer of conductive material on one of the substrates.

The substrate may also include power terminals, output terminals, and control terminals of the power die, the terminals being electrically connected to the patterned layer of conductive material.

Also disclosed is a method for manufacturing a power module according to the above description, the method comprising the following steps:

- two substrates are provided, each comprising a patterned layer of conductive material with contact pads and a layer of thermally conductive material,

- placing prepackaged power cells between two substrates, wherein each prepackaged cell includes a power die embedded in an electrically insulating core and connected to an outer layer of conductive material with contact pads, such that each prepackaged power cell The contact pads of the outer layer match the contact pads of the patterned layer of the substrate,

Wherein, the bonding material is present on the outer layer of the conductive material of the power unit or on the patterned layer of the conductive material,

- Bond the base plate and the prepackaged power unit together.

The method may include the further step of filling the remaining space between the substrates and the remaining space between the prepackaged power cells with a dielectric material.

The method may further include the step of mounting the at least one passive component on one of the patterned layers of conductive material of the substrate prior to the step of bonding.

The bonding material can be applied by screen printing or nozzle deposition.

The method may also include the preliminary step of manufacturing the prepackaged power cells such that all of the prepackaged power cells have the same thickness.

The power module according to the invention provides good heat dissipation due to the two substrates attached on opposite sides of the prepackaged power unit. Incorporating prepackaged power cells between the substrates allows ensuring the required electrical isolation, since the power cells act as spacers between the substrates, and also because they comprise a core of electrically insulating material.

Additionally, the use of prepackaged power cells enables connection to small electrical pads of the power die, as the contact pads of the substrate can be connected to enlarged contact pads of the power cells that function as fan-out packages.

To further enhance heat dissipation and electrical insulation, the spaces between the substrates of the power modules and the spaces between the prepackaged power cells can also be filled with electrically insulating and thermally conductive materials.

Other features and advantages of the present invention will become apparent from the following detailed description, given by way of non-limiting example with reference to the accompanying drawings.

Description of drawings

[figure 1]

Figure 1 is a schematic representation of a power module according to an embodiment of the invention.

[Figure 2a]

Figure 2a is a schematic representation of a power module according to another embodiment of the invention.

[Figure 2b]

Figure 2b is an enlarged view of a portion of the power module of Figure 2a.

[Figure 3a]

Figure 3a is a schematic diagram of a power module according to another embodiment of the present invention.

[Figure 3b]

Figure 3b is a schematic diagram of a power module according to another embodiment of the present invention.

[Figure 4]

Figure 4 schematically represents the main steps of a manufacturing method according to an embodiment of the present invention.

Detailed ways

Referring to Figure 1, a power module 1 according to an embodiment of the present invention will now be described.

The power module 1 includes two substrates 10 , each of which includes at least a thermally conductive material layer 11 (also an electrically insulating material) on which a conductive material patterned layer 12 is disposed. For example, the substrate 10 may be a direct bonded copper (DBC) substrate with a copper patterned layer 12 disposed on a ceramic plate 11 (eg, made of alumina) forming a thermally conductive and electrically insulating layer. According to other examples, the substrate 10 may be an insulated metal substrate (IMS) or an active metal braze (AMB) substrate.

The power module 1 also includes a plurality of prepackaged power cells 20 positioned between the two substrates 10, wherein the two patterned layers 12 of conductive material are positioned towards each other. Each prepackaged power cell 20 includes at least an electrically insulating core 21 in which at least one power die 22 is embedded. The power die may be a diode or transistor such as a MOSFET, JFET or IGBT, HEMT. In an embodiment, the power die 22 is made of a wide band gap semiconductor (ie, a semiconductor with a band gap in the range of 2-4 eV). For example, the power die can be fabricated in silicon carbide SiC or gallium nitride GaN.

The power die has electrical contacts 220 on opposite sides thereof (FIG. 2b). In an embodiment, the power die 22 is a diode and has two opposing electrical contacts 220 . In another embodiment, the power die 22 is a transistor and has three electrical contacts 220 including gate, source and drain or gate, emitter and collector depending on the type of transistor. Power die 22 may also have electrical contacts greater than three in number.

The electrically insulating core 21 of the prepackaged power unit 20 preferably has a low thermal resistance to provide better heat dissipation. The electrically insulating core 21 may be made of FR-4 glass epoxy, polyimide, or a ceramic such as HTCC (High Temperature Co-fired Ceramic) or LTCC (Low Temperature Co-fired Ceramic).

Additionally, the prepackaged power unit 1 also includes two outer layers 23 of conductive material (eg, copper) on opposing major surfaces of the electrically insulating core 21 . Thus, when the unit 1 is positioned between the substrates, the two outer layers 23 of the prepackaged power unit 1 are in contact with the corresponding conductive layers 12 of the substrate 10 .

Each outer layer 23 is patterned to match the pattern of the conductive layer 12 of the substrate 10 by etching or milling. To this end, each outer layer includes at least one contact pad 230 configured to mate with a contact pad (not shown) of one of the conductive layers when the prepackaged power cells are inserted between the substrates 10 . Furthermore, the contact pads 230 of the outer layer 23 or another contact pad of the same layer connected to the first contact pad and at the same potential are also connected to the corresponding electrical contacts 220 of the power die 22 .

Thus, a connection is established between the patterned conductive layer 12 of the substrate 10 and the power die 22 .

Thus, a power module may include a plurality of power dies 22 integrated in respective power cells (each power cell may include one or more power dies), and the power cells are connected according to the desired topology of the power module. For example, the power module may be an inverter or a DC/DC converter.

In the embodiment shown schematically in FIG. 1 , the contact pads 230 of the outer layers of the prepackaged power unit 1 are connected to the corresponding electrical contacts of the power die through vias 24 arranged in the electrically insulating core 21 .

In another embodiment, shown schematically in FIGS. 2a and 2b , the prepackaged power unit 1 further comprises at least two inner layers 25 of conductive material (eg copper), each layer being embedded in the electrically insulating core 21 and is positioned between the power die and the corresponding outer layer 23 . In this case, each inner layer 25 comprises at least one contact pad 250 connected to the corresponding electrical contact of the power die through a via 240 arranged in the electrically insulating core 21 , and the contact pad 230 of the outer layer 23 Connections to the contact pads of the respective inner layers 25 are made through other vias 241 arranged in the electrically insulating core 21 . Thus, the contact pads 230 of the outer layer 23 are connected to the electrical contacts of the power die through the inner layer 25 positioned therebetween.

Preferably, the surface area of the contact pads 230 of the outer layer is greater than the surface area of the power die electrical contacts 220 to which they are connected. Thus, the use of prepackaged power cells allows the contact surface area of the power die to be enlarged due to the enlarged contact pads 230 of the outer layers.

In embodiments where the prepackaged power cell 20 also includes an inner layer 25 of conductive material with corresponding contact pads 250, the surface area of the contact pads 250 of the inner layer 25 is greater than the surface area of the electrical contacts of the power die to which it is connected, and It may be smaller or have the same surface area as the contact pads 230 of the outer layer to which it is also connected.

Thus, the surface area between the electrical contacts of the power die and the contact pads 230 of the outer layer 23 is increased in any case.

This makes assembly and connection of the prepackaged power cell 2 to the substrate 10 easier, and also allows high power delivery to the die 22 .

In embodiments where the prepackaged power cell 20 includes an inner layer 25 of conductive material, the thickness of the outer layer may be greater than the thickness of the inner layer 25 (eg, at least five or ten times greater) in order to increase power transfer to the power die. According to non-limiting embodiments, the inner layer 25 may have a thickness of about 30-35 μm and the outer layer 23 may have a thickness of about 400 μm.

To accommodate adequate power transfer, the density of vias 240 connecting the power die electrical contacts to the inner layer contact pads may be at least 20 vias/mm ² (eg, 30 vias/mm ² ), such as vias The ratio of depth to drill diameter is 1:2.5.

Since the surface area of the contact pads of the outer layer is more important, the density of the vias 241 connecting the contact pads 250 of the inner layer and the contact pads of the outer layer may be equal to or lower than 30 vias/mm ² ; for example, vias The ratio of depth to drill diameter is 1:1.

The individual prepackaged power cells are attached to the base plate 10 of the module 1 by sintering, welding or liquid diffusion bonding techniques, as will be described in more detail below. Accordingly, the module 1 also includes a layer 30 of bonding material, such as solder paste, frit paste (eg, silver frit paste) or conductive paste, between the various outer layers 23 of the prepackaged power cells 1 and the conductive layer 12 of the substrate.

Since the thickness of the various power dies may be variable (the thickness being the distance between opposing electrical contacts of the power dies), all prepackaged power cells of the same power module 1 and containing power dies of varying thickness It preferably has the same thickness (measured as the distance between the outer layers 23 of the power cells). To accommodate thickness variations between power dies, the power cells may have an electrically insulating core of constant thickness determined to be sufficient to embed any power die and ensure adequate electrical isolation between the two substrates 10 .

Accordingly, the thickness of the electrically insulating core 21 is determined to be the minimum thickness that embeds the maximum thickness of the power die and provides electrical isolation between the substrates. Therefore, the thickness variation of the power die is compensated by the power cell, so the design and manufacture of the power module is easier.

The thickness of the power cells over the power dies also allows sufficient space to be created between the substrates 10 to provide the required electrical isolation between them.

As shown eg in Figures 3a and 3b, the power module 1 may also include additional passive components 40 (eg decoupling capacitors or gate resistors), which may be confined to one conductive layer 12 of one of the substrates.

The power module 1 also includes terminals, which may be part of one or more lead frames soldered on the conductive layer 12 . The terminals include a power terminal 50 , an output terminal 51 and a control terminal 52 of the die. The drivers can be soldered on one of the conductive layers and connected to the control terminals, or can be incorporated into the power cells along with the power dies they are intended to control in order to reduce the distance between the drivers and the power dies.

Finally, the power module 1 preferably also includes a dielectric material 60 filling the gaps between the substrates 10 and the gaps between the power cells and other components contained in the module. It can be FR-4 glass epoxy, parylene or silicone. Preferably, the dielectric material has a thermal conductivity greater than at least 1 W/(m.K) in order to enhance heat dissipation while also increasing electrical insulation between substrates 10 .

Referring to FIG. 4 , a method of manufacturing the above-described power module 1 will now be described.

The method includes: a first step 100 , providing two substrates 10 having a layer 11 of electrically insulating thermally conductive material and a patterned layer 12 of conductive material. Patterning is done by milling or etching. In an embodiment, at least one passive component, such as a capacitor, may be attached to one of the substrates, eg, by soldering. The installation of passive components may also be performed after step 200 is performed.

During a second step 200, a plurality of prepackaged power cells 20 according to the above description are positioned between the substrates such that each patterned layer 12 of the substrate 10 faces the outer layer 23 of the power cells and the contact pads of the patterned layer are aligned with The contact pads of the outer layer 23 are matched.

This step is easier to perform than positioning the power die directly on the substrate because the external contact pads of the prepackaged power cells (formed by the contact pads of the outer layers) are larger than the electrical contacts of the power die.

Preferably, prior to step 200, a bonding material 30 is applied on the contact pads of at least one of the outer layer 23 and the patterned layer 12 of the substrate. Preferably, the bonding material is applied to the contact pads of both the outer layer and the patterned layer 12 . The bonding material may be solder paste or frit paste or conductive paste. It can be applied by screen printing or nozzle deposition.

Then, the method includes the step 300 of bonding the substrate with the prepackaged power unit 20 . In the case of sintering, the two substrates can be pressed together by heat. Since the distance between the two substrates is ensured by the pre-packaged unit, there is no risk of tilting or pressure imbalance on the surface.

In the case of soldering, the surface tension caused by the phase transition of the solder paste can disrupt the positioning of the substrate with respect to the various contact pads of the prepackaged power cells. In this case, additional alignment pins (not shown) may be used to hold the assembly of the base plate and power unit in place.

A lead frame supporting the at least one terminal may also be attached to the at least one substrate by soldering or sintering for providing at least some of the module terminals. Bonding of the lead frame may also be performed before the prepackaged power cells are bonded to the substrate.

Optionally, during step 400, the remaining gaps between the substrates and the remaining gaps between components (power cells and/or passive components) may be filled by inserting a dielectric filler material in order to enhance electrical isolation between the substrates and heat dissipation.

Claims (15)

1. A power module comprising: - a first planar substrate and a second planar substrate, each substrate comprising a layer of thermally conductive material and a patterned layer of electrically conductive material, - a plurality of prepackaged power cells positioned between said first planar substrate and said second planar substrate, each prepackaged power cell comprising: ○ Electrically insulating core, o at least one power die embedded in said electrically insulating core, each power die having opposing electrical contacts, and o two outer layers of conductive material on opposite sides of the electrically insulating core, the outer layers being connected to respective patterned layers of conductive material of the planar substrate, wherein each outer layer of conductive material of the prepackaged power cell includes contact pads connected to corresponding electrical contacts of the power die through connections arranged in the electrically insulating core of the prepackaged power cell, The surface area of the contact pads is greater than the surface area of the electrical contacts of the power die to which they are connected. 2. The power module of claim 1, wherein each prepackaged power cell further comprises two inner layers of conductive material embedded in the electrically insulating core, each inner layer positioned between the power die and the Between the corresponding outer layers, the thickness of the outer layer is greater than the thickness of the inner layer, And the connection between the contact pads of the outer layer and the corresponding electrical contacts of the power die includes a first connection between the contact pads of the outer layer and the contact pads of the corresponding inner layer and A second connection between the contact pads of the respective inner layers and the respective electrical contacts of the power die. 3. The power module of claim 2, wherein the surface area of the contact pads of the inner layer of conductive material is greater than the surface area of a power die electrical contact to which it is connected. 4. The power module of claim 1 or 2, wherein the connection between the outer layer and the inner layer of the prepackaged power cell and the connection between the inner layer of the power die and the electrical contacts are arranged in the Through holes in the electrically insulating core. 5. The power module of any preceding claim, further comprising a layer of electrically and thermally conductive bonding material between each outer layer of the prepackaged power cell and the patterned layer of the substrate, the The bonding material is selected from the group consisting of solder paste, frit paste or conductive paste. 6. The power module of any preceding claim, further comprising a dielectric material filling the spaces between the substrates and the spaces between the prepackaged power cells. 7. The power module of any preceding claim, wherein at least two power dies contained in different prepackaged power cells have as one of the electrical contacts on opposite sides of the power dies Different thicknesses measured at the maximum distance between and the corresponding prepackaged power cells have equal thicknesses measured between their respective two outer layers of conductive material. 8. A power module according to any preceding claim, wherein the planar substrate is a direct bonded copper substrate, an insulated metal substrate or a reactive metal brazed substrate. 9. The power module of any preceding claim, further comprising at least one passive component mounted on the patterned layer of conductive material of one of the substrates. 10. The power module of any preceding claim, wherein the substrate further comprises power, output and control terminals of the power die, the terminals being electrically connected to the conductive material Composition layers. 11. A method for manufacturing a power module according to any preceding claim, the method comprising the steps of: - providing two substrates, each comprising a patterned layer of electrically conductive material with contact pads and a layer of thermally conductive material, - placing a prepackaged power unit between the two substrates, wherein each prepackaged unit comprises a power die embedded in an electrically insulating core and connected to an outer layer of conductive material with contact pads, such that each prepackaged unit the contact pads of the outer layer of the packaging power unit are matched with the contact pads of the patterned layer of the substrate, wherein the bonding material is present on the outer layer of the conductive material of the power unit or on the patterned layer of the conductive material, - Bonding the base plate and the prepackaged power unit together. 12. The method of claim 11, further comprising the step of filling the remaining space between the substrates and the remaining space between the prepackaged power cells with a dielectric material. 13. The method of claim 11 or 12, further comprising the step of mounting at least one passive component on one of the patterned layers of conductive material of the substrate prior to the bonding step. 14. The method of any one of claims 11 to 13, wherein the bonding material is applied by screen printing or nozzle deposition. 15. The method of any one of claims 11 to 14, further comprising the preliminary step of manufacturing the prepackaged power cells such that all of the prepackaged power cells have the same thickness.

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